1. Field of the Invention
The present invention pertains to a system for irradiating patients with charged particles and to a method for monitoring the system.
2. Description of the Related Art
Irradiation systems that irradiate using charged particles offer considerable advantages over conventional irradiation systems which work with x-rays or photon beams. These advantages include the greater accuracy with which the administered dose can be targeted and the decrease in the severity of the side effects on healthy tissue.
Conventional photon beams penetrate the body, but they are also absorbed during their interaction with the molecules of the body and thus undergo a continuous loss of intensity. The maximum dose is present just under the skin, as can be seen from dose-distribution curve A in FIG. 1. This effect is based on the “recruiting” of stray radiation, which occurs after the beam has reached a point just under the skin. As the beam proceeds onward toward the tumor area Z, the radiation dose then decreases in accordance with an exponential curve. A deep tumor thus receives less of the dose than the healthy tissue located in the path of the beam in front of the tumor, and even the organs situated behind the tumor still receive a considerable dose of radiation.
In contrast, charged particles such as protons and heavy ions lose relatively little energy at first, i.e., just after entering the body, but then they are decelerated by repeated interactions with matter (see FIG. 2). The slower the particles become, the more energy they give off and the more they are decelerated. This leads to an “energy explosion” at the end of the particle path, the so-called “Bragg peak” (dose-distribution curve B in FIG. 2). The dose of charged particles in front of the tumor is much smaller than that delivered by irradiation with photons, and the greater part of the dose is thus concentrated in the tumor. In the case of protons, the patient actually remains free of the radiation behind the tumor. Through proper control of the generated particle velocity in coordination with the scanning method, this physical phenomenon makes it possible to deliver the dose into the tumor three-dimensionally. The Bragg peak is so sharp that it must be moved not only laterally over the tumor but also in the depthwise direction through variation of the particle velocity, as can be seen in FIG. 3, which shows a Bragg plateau C.
A preferred scanning method is the so-called “raster scanning method”, in which the Bragg peak of the beam of charged particles travels across the tumor under computer control with millimeter accuracy in a three-dimensional grid preestablished by various diagnostic and irradiation planning procedures. According to this method, the beam, which typically has a diameter of 10 mm FWHM (full width at half maximum), is aimed at the individual raster points one after the other, each point thus being exposed typically for 60-90 seconds to the specifically selected dose in each irradiation session. A patient treatment takes place over the course of several of these irradiation sessions on successive days.
Especially in cases of tumors located close to healthy structures vital to life and of very deep tumors, which in many cases cannot be treated with conventional photon beam therapy at all because of the unavoidable, undesirable damage to healthy tissue or which in other cases cannot be treated with conventional photon beam therapy except at very high risk, irradiation with charged particles represents a significant advance in the area of cancer treatment.
When charged particles are used to irradiate patients according to the raster scanning method, the sharp, concentrated dose distribution of the pencil beam explained above and the associated accurately targeted three-dimensional irradiation also impose additional requirements on the accuracy which must be maintained.
For this reason, systems for irradiating patients with charged particles comprise a plurality of safety devices for checking the therapeutic planning data and the functionality of the system, so that incorrectly calculated therapy planning data and data transmission errors between the individual components of the system can be minimized. Thus even isolated data corruption (random errors), which can also have highly disadvantageous effects on the results of the irradiation treatment, can be excluded even more effectively.